SnO2 nanoparticles have frequently been reported as effective electrocatalysts for CO2 electroreduction to formate. However, in the literature, there is little knowledge of SnO2 nanoparticles that guarantee superior electrocatalytic performance. Hence, in this study, several SnO2 nanoparticles are compared with respect to their material properties, and correlations to the electrocatalytic performance are established. For comparison, three custom-made SnO2-electrocatalysts were prepared, reproducing frequently cited procedures in literature. Based on the comparison, it is found that hydrothermal, sol-gel, and solid-state synthesis provide quite different electrocatalysts, particularly in terms of the particle size and crystal lattice defect structure. Desirably small nanoparticles with a comparatively high number of lattice defects are found for the nanoparticles prepared by hydrothermal synthesis, which also provide the best electrocatalytic performance in terms of Faradaic efficiency for the electroreduction of CO2 to formate. However, despite the considerably smaller surface area, the commercial reference also provides significant electrocatalytic performance, e.g., in terms of the overall produced amount of formate, which suggests a surprisingly high surface area-specific activity for this material that is low on defects. Thus, defects do not appear to be the preferred reaction site for the CO2 electroreduction to formate on SnO2 in this case.